1. Technical Field
This invention relates generally to a sensor for the measurement of weak magnetic fields.
More specifically, this invention relates to a fluxgate sensor for the detection of small magnetic gradients in the presence of a much larger, static magnetic field.
2. Description of Related Art
One of the most common uses of fluxgate sensors is in magnetic detection devices that locate buried or otherwise hidden pipes and other artifacts by detecting the small magnetic gradients produced by those objects. Traditional fluxgate sensors use thin strips of permalloy, or other saturable ferromagnetic material, that is formed into a toroidal solenoid assembly. The material is driven into magnetic saturation by passing a high amperage alternating current through the solenoid to produce a time varying change in the effective permeability of the space inside the solenoid. That time varying change in permeability causes a time varying change in the static ambient magnetic field near the solenoid. A time varying voltage that is proportional to the static ambient magnetic field can then be produced using a simple pickup coil that would not respond to the static magnetic field alone.
Typical fluxgate sensors, such as for example the Schonstedt scroll sensor that is described in his U.S. Pat. No. 2,916,696, use very thin permalloy tape that is wound on a nonmagnetic base to form the sensor core. An excitation solenoid coil is then wound around that core. The winding of the permalloy core and of the excitation solenoid are hand operations with small associated inaccuracies that cause the sensors to vary in properties, one to the next. In addition, the permalloy tapes used to form the sensor core necessarily have a beginning and an end causing the cores to show asymmetry in at least one axis and a discontinuity that is harder to saturate than is the main core body. Those conditions result in a vector magnetic field measuring device having a magnetic axis that is different from its mechanical axis, with the difference dependent on external factors such as excitation drive level.
Magnetic gradient detection devices ordinarily employ two fluxgate sensors that are spaced apart and accurately aligned one to the other. Sensor misalignment is usually the greatest source of error in gradient detection devices that use conventional fluxgate sensors. The signal output from one sensor is subtracted from the output of the other sensor to cancel that portion of the signal attributable to any uniform magnetic field present at the measuring location but to respond to any difference in field between the two sensors. These devices are used to detect magnetic gradients as small as 10 gammas per foot in the presence of an ambient earth magnetic field ranging from about 25,000 gammas to about 70,000 gammas. A physical misalignment between sensors of arcsin (10 gamma/70,000 gamma), which is less than one one-hundredth of a degree, will cause a change in response of the detection device as it is moved that responds like a 10 gamma gradient.
It is evident that a fluxgate sensor that overcomes the problems inherent in the design of conventional sensors would be a useful and desirable improvement in the art. This fluxgate sensor of this invention achieves those ends.
The fluxgate sensor of this invention is formed as a laminate that includes a flat, saturable magnetic core of generally oval, elliptical or circular shape bonded between a pair of non-conductive, substrate members. The exterior of each substrate member includes a multi-element metallic track that is shaped to form a toroidal excitation winding about the core by means of multi-point electrical connections through the substrate members. At least one of the substrate members includes indicia defining the magnetic axis and the mechanical axis of the sensor so as to allow precise alignment of one sensor with another.
Hence, it is an object of this invention to provide an improved fluxgate sensor and to provide as well a method for its convenient manufacture.
Another object of this invention is to provide a fluxgate sensor that can be accurately aligned in a spaced apart relationship with another fluxgate sensor.
Other objects of this invention will become apparent from the following description of a preferred embodiment of the invention.